Abstract
WITTING, BROOKE ELLEN. Evaluation of Floral Habitat as a Food Source for Natural Enemies of Insect Pests in North Carolina. (Under the direction of David B. Orr and H. Michael Linker).
A field study was conducted in 2004 and 2005 to observe flower- feeding of potential
beneficial insectary plants by insects. Sixteen flower species were individually observed
once weekly for two minutes beginning between 12 and 1 pm in 2004. Five species were
observed twice weekly beginning at 9:30 am and 12 pm in 2005. Insects were identified to
family level and analyzed by feeding guild. In both years, predators were observed feeding
from fennel (Foeniculum vulgare P. Mill.) flowers in greater abundance than from any other
flowers observed. Fennel also was fed upon most often by parasitoids in 2005. Pollinators
were observed feeding most often from Indian blanket (Gaillardia pulchella Foug.) in 2004
and from black-eyed Susan (Rudbeckia hirta L.) and buckwheat (Fagopyrum esculentum
Moench) in 2005. In both years, herbivorous crop pests, deleterious and non-crop
parasitoids, and deleterious predators were not significantly affected by flower species.
A field study was conducted in August 2005 to determine the relative attractiveness
of floral habitat to three families of microhymenopteran egg parasitoids: Mymaridae,
Scelionidae, and Trichogrammatidae. Habitat plants were yarrow (Achillea millefolium L.),
celosia (Celosia cristata L.), buckwheat, fennel, daisy (Leucanthemum x superbum (J. W.
Ingram) Be rg. ex Kent.), and black-eyed Susan. Non- flowering crabgrass (Digitaria sp.
Haller) served as a control. Sticky traps were used to monitor microhymenoptera and were
placed at three heights: flower height, 0.5 times flower height, and 1.5 times flower height.
Flower heads were removed from half of each plot and traps were placed in the center of
three families of microhymenoptera but flower removal only affected scelionids. At flower
height, scelionids were trapped in greater abundance in celosia plots at flower height in
flowers-present versus flowers-removed treatments. Trichogrammatids were trapped in
greatest abundance at 0.5 times flower height in un- mowed crabgrass plots and mymarids
were most abundant at 0.5 times flower height in black–eyed Susan plots. Our results
indicate that habitat plantings may attract microhymenoptera but that flowers themselves do
not appear to be responsible for this attraction.
A combined laboratory and field study was conducted to determine the effect of
different food sources on the longevity and fecundity of Trichogramma exiguum Pinto &
Platner and the longevity of Cotesia congregata (Say). Newly eclosed (<12 h) female wasps
were provisioned with one of two treatments: fennel or buckwheat flowers, or one of two
controls: honey or water. Wasps were monitored daily until all had died. Fecundity of T.
exiguum was monitored using Ephestia kuehniella Keller egg cards. Longevity was greatest
in T. exiguum provisioned with honey and in C. congregata provisioned with buckwheat
flowers. Buckwheat provisioned T. exiguum exhibited greater longevity than those provided
fennel. Longevity of C. congregata provisioned with fenne l and honey was approximately
equal. Water provisioned T. exiguum and C. congregata exhibited the shortest longevity.
Total fecundity was greatest in T. exiguum provisioned with honey or buckwheat. Average
female to male ratio over the lifetime of each female was greatest in T. exiguum provisioned
with water alone, likely because of sperm limitation in wasps exhibiting greater longevity.
Total average number of female offspring produced was greatest in T. exiguum provided
honey or buckwheat flowers although no difference in total female offspring were observed
provisioning T. exiguum with honey and buckwheat flowers caused greater longevity, total
fecundity, and lifetime production of female offspring than water alone. Buckwheat flowers
EVALUATION OF FLORAL HABITAT AS A FOOD SOURCE FOR NATURAL ENEMIES
OF INSECT PESTS IN NORTH CAROLINA by
BROOKE ELLEN WITTING
A thesis submitted to the Graduate Faculty of North Carolina State University
In partial fulfillment of the Requirements for the degree of
Master of Science
ENTOMOLOGY Raleigh
2006
APPROVED BY:
________________________________ ________________________________ Dr. David B. Orr Dr. H. Michael Linker
Co-Chair of Advisory Committee Co-Chair of Advisory Committee
DEDICATION
I am pleased to dedicate this thesis to my mom, Sylvia Lynn Witting, whose stubborn
BIOGRAPHY
Brooke Ellen Witting was born in Norman, Oklahoma in 1978. She moved to
Durham, North Carolina with her family in 1981. Brooke attended Warren Wilson College
in Swannanoa, NC and received her B.A. in Environmental Studies with a minor in Biology
in 2000 under Dr. Louise Weber. It was at WWC that she discovered her love for the life
sciences and entomology. In 2004 she began work on a Master’s of Science degree at North
Carolina State University in the Department of Entomology under the direction of Drs. David
ACKNOWLEDGEMENTS
I would like to extend my sincere thanks to Dr. David Orr and Dr. H. Michael Linker
for accepting me into the graduate program when I felt I would never overcome the obstacles
to get there. I would especially like to thank Dr. Orr for teaching me about experimental
design and creative thinking. My warm thanks go to Dr. Linker for allowing me to tag-along
while he conducted his research when I was an intern at the Center for Environmental
Farming Systems. I learned more on those field days than I could ever learn in the
classroom. I also would like to thank Dr. Cavell Brownie for her help with statistical
analysis and Dr. John Dole for his horticultural expertise.
I am grateful to Lisa Jackson, Meg Perry, Aaron Thomas, Evan Palmieri, Oliver
Freeman, Derek Frank, Alexandre Roberts, Jenny Pate, Ryan Hinson, and Sergio Hernandez
for helping collect data and maintain plots under often grueling conditions. My special
thanks are extended to Lisa Forehand for her taxonomic expertise in the microhymenoptera
and to Mary Kroner for tirelessly accompanying me during my first field season.
Finally, I would like to thank Dr. Louise Weber for igniting the spark that fueled my
interest in entomology. I will never forget her first lecture as she described spending hours
examining insect boxes and being astounded by the diverse and beautiful world of insects. It
TABLE OF CONTENTS
Page
LIST OF TABLES……….………..…...vi
LIST OF FIGURES………..………..vii
I. OBSERVATIONS OF INSECT FLOWER-FEEDING……….1
Abstract………...2
Introduction……….3
Materials and Methods………...….6
Results………10
Discussion……….….14
References cited……….17
II. RELATIVE ATTRACTIVENESS OF HABITAT PLANTINGS TO MICROHYMENOPTERA………24
Abstract………..……25
Introduction………26
Materials and Methods……….…..28
Results………30
Discussion………..32
References cited………...…..37
III. EFFECTS OF FOOD TYPE ON LONGEVITY AND FECUNDITY OF TRICHOGRAMMA EXIGUUM AND LONGEVITY OF COTESIA GLOMERATA………40
Abstract………...……41
Introduction……….42
Materials and Methods………45
Results……….51
Discussion………...52
References cited………...55
LIST OF TABLES:
Page
1.1 Plant species observed in each beneficial insect habitat flower
strip. Goldsboro, NC………..20
1.2 List of insect families by feeding guild………...21
1.3 Mean ± SD number insects in each feeding guild observed
feeding from flowers. Goldsboro, N.C. 2004………22
1.4 Mean ± SD number insects in each feeding guild observed
feeding from flowers. Goldsboro, N.C. 2005………23
2.1 Mean ± SD number of parasitoids caught on yellow sticky traps placed at three different heights in plots with flowers
present or mechanically removed from five plant species……….39
3.1 Mean ± SD longevity of T. exiguum and C. congregata provided
different food sources………62
3.2 Mean ± SD number of offspring, females, and percent females
LIST OF FIGURES:
Page
3.1 Diagram of experimental cage………..………..57
3.2 Mean daily ± SD number of offspring produced by T. exiguum
provisioned with buckwheat flowers and water in a laboratory
study ………...58
3.3 Mean daily ± SD number of offspring produced by T. exiguum
provisioned with fennel flowers and water in a laboratory study……..59
3.4 Mean daily ± SD number of offspring produced by T. exiguum
provisioned with honey and water in a laboratory study………..60
3.5 Mean daily ± SD number of offspring produced by T. exiguum
Observations of Insect Flower-Feeding
BROOKE ELLEN WITTING
Department of Entomology, College of Agriculture and Life Sciences
Abstract
Habitat plantings may be used to increase diversity of natural enemies to enhance
biological control of agricultural pests by providing nectar and pollen, an appropriate
microclimate, or hosting alternative prey. This study was conducted in 2004 and 2005 to
observe flower-feeding of potential beneficial insectary plants by insects. Sixteen flower
species were individually observed once weekly for two minutes beginning between 12 and 1
pm in 2004. Five species were observed twice weekly beginning at 9:30 am and 12 pm in
2005. Insects were identified to Family level and analyzed by feeding guild. In both years
more predators were observed feeding from fennel (Foeniculum vulgare P. Mill.) flowers
than from any other flowers. Fennel also was fed upon most often by parasitoids in 2005.
Pollinators were observed feeding most often from blanket flower (Gaillardia pulchella
Foug.) in 2004 and from black-eyed Susan (Rudbeckia hirta L.) and buckwheat (Fagopyrum
esculentum Moench) in 2005. In both years, herbivorous crop pests, deleterious and
non-crop parasitoids, and deleterious predators were not significantly affected by flower species.
Introduction
Insects and flowering plants are believed to have relied on one another for the last
125 million years. Palynivory (feeding on pollen) is considered to be the evolutionary
forerunner to pollination and was followed by nectarivory where plants ensured their
reproduction by enticing pollinators with nectar rewards (Labandeira 1998). While less
frequently noted, plants may attract insects using nectar for another reason. By luring
predatory insects with sweet secretions, plants can potentially encourage predators to feed on
herbivorous insects (Wäckers 2005).
Plants can assist natural enemies by providing appropriate microclimates, food
resources, such as nectar and pollen, or by hosting alternative prey (Landis et al. 2000).
While many natural enemies are carnivorous as larvae, the adults are often omnivorous or
herbivorous and rely on plant foods to promote increased longe vity and fecundity (Jervis and
Kidd 1986; Cortesero et al. 2000; Wäckers 2005). The ‘enemies hypothesis’ (Root 1973)
implies that natural enemies are more effective at reducing crop pest numbers in diverse
rather than simple habitats. In modern cropping systems, plant diversity tends to be low,
reducing plant resources such as sugar, which may impact beneficial insects. Habitat
management is a type of conservation biological control that employs the use of plant
resources to enhance the effectiveness of natural enemies and can be an important tool in
suppressing agricultural pest insect populations by increasing diversification of plants in
agricultural systems (reviewed by Coll 1998).
Plant-provided resources can increase effectiveness of natural enemies by generating
greater longevity, fecundity, or host-searching ability. The effectiveness of insectary habitat
about half of the studies comparing diversified cropping systems to monocultures have
yielded positive results in terms of reduced pest numbers (Heimpel et al. 2005). Failures in
the field may be caused in part to plants’ varied abilities to provide natural enemies with food
resources due to plant physiology and morphology. Many factors influence the suitability of
floral habitat as food sources to natural enemies including availability of flowers in time and
space, floral architecture, floral odor, and nutritional composition of nectar and pollen
(Wäckers 2005). For example, buckwheat (Fagopyrum esculentum Moench) flowers are
readily fed upon by many natural enemies (English- Loeb 2003); however, nectar production
ceases in the afternoon (Olson et al. 2005). Patt et al. (1997) found that floral architecture
and odor played important roles in the foraging efficiency of two parasitoids (Hymenoptera:
Eulophidae). Analysis of gut sugars of parasitic ichneumonoid and chalcidoid wasps showed
significantly higher amounts of fructose, a sugar not naturally present in insect bodies, in
wasps collected from flowering buckwheat borders than from soybean borders (Lee and
Heimpel 2003). Wäckers (2004) screened eleven species of flowering plants for suitability
as food sources to an ichneumonid and two braconid parasitoids. Only four plant species
were found to be attractive, while three plant species were actually determined to be
repulsive.
It is generally accepted that natural enemies forage effectively on non-specialized
flowers such as composites and umbels which contain compact groups of small florets with
accessible nectaries (Proctor et al. 1996). Field observations have shown that natural
enemies exhibit preferential feeding behavior to various species of flowering plants. In a
study conducted by Colley and Luna (2000), coriander (Coriandrum sativum L.), fennel
elicited the greatest number of feeding visits from beneficial hoverflies visiting eleven
flowering plant species. Carreck and Williams (1997) recorded insect visits to individual
flowering species of two commercial flower mixes and found Phacelia tanacetifolia Benth to
be most attractive to hoverflies and hymenoptera. However, the hymenoptera observed were
predominately members of the family Apidae, not parasitoids. Lövei et al. (1993) observed
feeding by hoverflies from several species of flowering plants and determined that coriander
provided food resources to the greatest number of hoverflies of the plants observed.
Al-Doghairi and Cranshaw (1999) observed insect visits to 150 plant species in 37 families and
found that members of Asteraceae, the aster family; Apiaceae (formerly Umbelliferaceae),
the carrot family; Brassicaceae, the mustard family; Lamiaceae, the mint family;
Scrophulariaceae, the figwort family; and Crassulaceae, the stonecrop family received the
most visits by natural enemies.
This study was designed to determine which flowers attracted the greatest numbers of
parasitoids and predators of crop pests in North Carolina. The observational studies
mentioned previously were conducted in Oregon, the United Kingdom, New Zealand, and
Colorado respectively. To our knowledge, few if any observational studies of flower- feeding
by natural enemies have been conducted in the southeastern United States. Forehand (2004)
recorded abundance of natural enemies collected from flowering habitat in North Carolina
using a vacuum sampler; however direct observations of flower- feeding by natural enemies
were not made.
We were also interested in recording the numbers of herbivorous crop pests feeding
from flowers. One risk associated with placing flowering plants near crop fields is that pest
found that while buckwheat and coriander flowers increased longevity of an encyrtid
parasitoid, flowers also increased longevity and fecundity of the parasitoid’s herbivorous
host. Pest numbers and crop damage in the field were also amplified as proximity to
flowering habitat increased.
Finally, it is important to quantify members of other feeding guilds in addition to
beneficial parasitoids, predators, and crop pests. Many insects can be observed feeding from
flowers that to farmers may appear to be beneficial. These insects include hymenoptera that
are pollinators, predators of beneficial spiders (e.g. members of Pompilidae) and parasitoids
of pollinators or natural enemies (e.g. members of Chrysididae). The previously mentioned
study by Carreck and Williams is a good example of documentation of hymenopteran
pollinators rather than predators and parasitoids being considered beneficial insects.
Stephens et al. (1998) provide an important case where numbers of Anacharis sp.
(Hymenoptera: Figitidae), a parasitoid of the beneficial brown lacewing, were increased in
orchards sown with buckwheat than in herbicide treated control plots. We hope that this
study can elucidate the preferences of different feeding guilds to floral habitat to provide
growers with a preliminary recommendation of insectary habitat for natural enemies in North
Carolina.
Materials and Methods
Research site. This study was conducted at the Center for Environmental Farming Systems near Goldsboro, N.C. on the Small Farm Unit. The Small Farm Unit is a highly
diverse, organic farm approximately 6.07 ha in size. A wide variety of commodities are
crops. Some livestock production, including chickens, turkeys, and goats also occurs on the
Small Farm Unit.
Experimental design. Observational data were collected from three flower strips in three distinct locations on the Small Farm Unit that were established the previous year.
Flower strips were separated by an average distance of 48.2 m. For all studies, flower strips
measuring approximately 56.4 x 2.7 m were divided into 6.1 x 2.7 m plots.
In 2004, each flower strip contained five plots, three of which were commercially
available beneficial insectary plantings and two that contained pure stands of fennel and
buckwheat (Table 1.1). Greenhouse grown plants were transplanted using a grid to achieve
an ideal plant community according to seed companies’ instructions in a complete block
design with selective placement of plots (Forehand 2004).
In 2005, flower strips contained seven plots laid out using a complete block design
with selective placement of plots. Fennel, daisy (Leucanthemum x superbum (J. W. Ingram)
Berg. ex Kent.), yarrow (Achillea millefolium L.), and black-eyed Susan (Rudbeckia hirta L.)
were planted because natural enemies were observed feeding from these plants most often
during the 2004 study. Celosia (Celosia cristata L.) appeared to attract and feed a large
number of natural enemies when observed anecdotally. Buckwheat was chosen because of
its prevalence in scientific literature as being an insectary plant attractive to natural enemies
(Colley and Luna 2000; Irvin et al. 2000; Stephens et al. 1998), although for the most part,
results have been variable (Irvin et al. 1999; Berndt et al. 2002).
Plant management. In 2004 and 2005, plants were watered as needed and weeds were managed with hand-weeding inside plots and mechanical mowing around plots. In
densities were high enough that a pure stand had already been obtained. All other plants
were either transplanted or directly seeded into plots. In 2005, flower strips each contained
seven plots. Plots were planted with greenhouse- grown celosia, daisy, and black-eyed Susan
transplants on 25 May, 2005. Fifty- four plants of each species were planted per plot in three
rows with 30.5 cm between each plant and 46 cm between each row using hand trowels and
bulb diggers. Buckwheat was directly seeded into plots at a rate of 56.04 kg/ha and raked in
using a steel rake. Buckwheat seed was purchased from Jeffrey's Seed Co. (1608 US 117
South, Goldsboro, NC 27503). The remaining seeds were purchased from Germania (5978 N
Northwest Hwy, PO Box 31787, Chicago, IL 60631-0787) (See Table 1.1 for cultivars).
Celosia, black-eyed Susan, and daisy transplants were grown in the Biological
Control Greenhouse at North Carolina State University, Raleigh, NC. Plants were started in
96-cell round plug trays (3.8 by 3.9 cm, Hummert International, 4500 Earth City
Expressway, Earth City, MO 63045) filled with moistened Metro-Mix 200 potting soil
(Scotts-Sierra Horticulture Products Co., The Scotts Company, 1411 ScottsLawn Rd.,
Marysville, OH 43041) on 25 and 28 March, 2005. Four trays were planted per species with
two seeds planted per cell thinned to one plant per cell. Plants were grown in a greenhouse
with a heating set point of 21.1º C and a ventilation set point of 26.7º C. Plants were watered
as need with a misting bed and/or hand watering. Trays were placed under high intensity
metal halide lights with an 11 h photophase. The photophase was extended to 16 h on 22
April, 2005. When roots were established and the aboveground portion was of sufficient
size, plants were transplanted to 473 ml plastic cups (Kmart Corporation, Troy, MI 48084)
with a drainage hole drilled in the bottom using a 1.3 cm drill bit. Prior to transplanting,
were covered with woven black plastic ground cover (Wyatt-Quarles Seed Company, 730
Hwy 70 West, Garner, NC 27529) secured with landscape anchor pins (DuPont™ Garden
Products™, Chestnut Run Plaza, Bldg. 728, PO Box 80728, Wilmington, DE, 19880-0728)
to suppress weeds and preserve soil moisture.
Sampling. In both years, one observation of insect flower-feeding per plant species was made in each replicate on each sampling date. Observations of insect feeding were
conducted on seven dates in 2004 (2 June, 9 June, 24 June, 8 July, 14 July, 22 July, and 4
August) and on thirteen dates in 2005 (21 June, 24 June, 28 June, 1 July, 5 July, 12 July, 15
July, 18 July, 2 August, 5 August, 9 August, 12 August, and 16 August). Observations in
2004 began between 12 and 1 pm. This time was chosen after performing a daylong
observation of insect activity on 31 May, 2004 from dawn to dusk where we found the
greatest amount of activity to occur midday. Observations were made at 9:30 am and 12:00
pm in 2005. The 9:30 observation was added due to low numbers of insects found feeding
midday on buckwheat in 2004, presumably because peak nectar production in buckwheat
occurs in the mo rning (Olson et al. 2005; Free 1993).
A single observer called out identified insects to a recorder who also kept time. This
approach allowed the observer to watch flowers for the prescribed period without
interruption. For a single observation, the observer constantly scanned an approximately 0.3
m2 area of actively blooming flowers of a single plant species for two minutes. Insects observed directly feeding from flower heads were recorded to family level. Feeding was
considered to be direct application of the insects’ mouthparts to the area of the plant
producing nectar and/or pollen or apparent application of the mouthparts to this region
from flower to flower within the area of observation were counted once. Insects that left the
area and returned were counted a second time, similar to methods described by Colley and
Luna (2000).
All insects that were too small to be identified in the field were removed with an
aspirator and transferred to a vial containing 50% ethanol and returned to the laboratory for
identification. Preliminary identification of specimens from each insect family was
performed by Dr. David Orr. Mr. David Stephan verified identification and specimens were
placed in the NCSU museum as vouchers.
Data analysis. Insects observed feeding on flowers were grouped according to feeding guilds (Table 1.3). The number of insects observed feeding at each plant species
were square root transformed then analyzed using general linear and mixed models for each
feeding guild (PROC GLM, PROC MIXED, SAS Institute 2003). Plant species that
flowered in only one replicate or received no feeding visits from members of a specific
feeding guild were omitted prior to analyses to avoid skewing results. Dates of observations
that fell within the same week in 2005 were combined prior to analyses to reduce imbalance
in data due to differences in blooming period among plant species.
Results
In 2004, numbers of parasitoids, predators, pollinators, and non-crop herbivores
observed were significantly affected by flower species (F = 6.60, df = 3, 5, P = 0.0344; F =
10.45, df = 9, 16, P < 0.0001; F = 12.43, df = 9, 16, P < 0.0001; F = 4.05, df = 9, 16, P =
0.0073) (Appendix 1.1). Herbivorous crop pests, deleterious and non-crop parasitoids, and
deleterious predators were not significantly affected by flower species (F = 1.57, df = 9, 16,
2, P = 0.2506). In 2005, flower species significantly affected the numbers of parasitoids,
non-crop parasitoids, predators and pollinators (F = 41.79, df = 2, 4, P = 0.0021; F = 27.45,
df = 4, 8, P < 0.0001; F = 9.08, df = 4, 8, P = 0.0045) but not non-crop herbivores,
herbivorous crop pests, deleterious parasitoids, or deleterious predators (F = 1.23, df = 3, 6, P
= 0.3773; F = 2.67, df = 4, 8, P = 0.1104; F = 2.86, df = 3, 6, P = 0.1267; F = 9.74, df = 1, 2,
P = 0.0891). Pollinators and deleterious parasitoids were affected by time of day
observations were made (F = 12.69, df = 1, 10, P = 0.0052; F = 9.86, df = 1, 10, P = 0.0105)
while parasitoids, non-crop parasitoids, predators, deleterious predators, non-crop herbivores
and herbivorous crop pests were not (F = 1.16, df = 1, 6, P = 0.3235; F = 3.19, df = 1, 10, P
= 0.1042; F = 0.16, df = 1, 10, P = 0.6966; F = 0.70, df = 1, 4, P = 0.4487; F = 0.50, df = 1,
8, P = 0.4994; F = 0.95, df = 1, 10, P = 0.3528). The interaction between time of day and
flower species significantly affected pollinators, deleterious parasitoids, and predators (F =
16.58, df = 4, 10, P = 0.0002; F = 7.07, df = 4, 10, P = 0.0057; F = 8.85, df = 4, 10, P =
0.0025) but not parasitoids, non-crop parasitoids, deleterious predators, non-crop herbivores
and herbivorous crop pests (F = 1.00, df = 2, 6, P = 0.4207; F = 0.98, df = 4, 10, P = 0.4620;
F = 0.18, df = 1, 4, P = 0.6907; F = 0.94, df = 3, 8, P = 0.4664; F = 1.10, df = 4, 10, P =
0.4062).
In 2004, overall parasitoid feeding was low (Table 1.3). Parasitoids were only
observed feeding from four flowers: celery (Apium graveolens L.), daisy, fennel, and yarrow.
Of these flowers, significantly more parasitoids were found feeding from celery. The
remaining flowers did not differ in the numbers of parasitoids observed feeding from them.
However, because celery was observed on relatively few occasions, results are not highly
from fennel in higher numbers than from any of the other plant species observed (Table 1.4).
Approximately equal numbers of parasitoids fed from yarrow, celosia, buckwheat, and
black-eyed Susan.
In 2004, predators were observed feeding in significantly higher numbers from fennel
than the remainder of the flowers observed (Table 1.3). Predators fed from celery and yarrow
at higher levels than from clover (Trifolium repens L.), blanket flower (Gaillardia pulchella
Foug.), California poppy (Eschscholzia californica Cham.), and tickseed (Coreopsis
lanceolata L.). In 2005, significantly more predators fed from fennel than the other flower
species regardless of time of day (Table 1.4). Buckwheat was fed upon to a lesser degree
than fennel, however significantly more predators were present on buckwheat at 9:30 than at
12:00.
Flowers in this study varied greatly in the numbers of pollinators that fed from them.
In 2004, higher numbers of pollinators were found feeding from blanket flower, although
numbers did not significantly differ from pollinators feeding from tickseed (Table 1.3).
Numbers of pollinators feeding from tickseed, fennel, yarrow, daisy, black-eyed Susan, and
California poppy were approximately equal while celery, clover, and buckwheat were fed
upon least. In 2005, more pollinators were observed feeding from black-eyed Susan and
buckwheat than all other plant species (Table 1.4). More pollinators were observed at both
black-eyed Susan and buckwheat at 9:30 than at 12:00.
In 2004, no n-crop herbivores fed most from celery flowers (Table 1.3). Yarrow was
fed upon more frequently than California poppy but no significant difference was found
among the remainder of the flower species. Non-crop herbivores were not significantly
The effects of replication and date on numbers of insects feeding from flowers in
2004 were significant for parasitoids (F = 3.98, df = 2, 11, P = 0.0383; F = 14.48, df = 6, 7, P
< 0.0001). Date also significant ly affected non-crop herbivores (F = 2.37, df = 6, 13, P =
0.0401). However, this was probably due to unevenness in the data as parasitoids and
non-crop herbivores were found feeding most often from celery, which was present in only two of
the three replicates for two weeks. Replication did not effect deleterious or non-crop
parasitoids, deleterious predators, predators, pollinators, non-crop herbivores, or herbivorous
crop pests (F = 0.54, df = 2, 13, P = 0.5883; F = 0.59, df = 2, 13, P = 0.5568; F = 3.33, df =
2, 10, P = 0.0779; F = 0.23, df = 2, 17, P = 0.7948; F = 2.91, df = 2, 17, P = 0.0619; F =
0.87, df = 2, 17, P = 0.4228; F = 0.50, df = 2, 17, P = 0.6100). Date played a significant role
in the number of pollinators and non-crop parasitoids found feeding from flowers (F = 9.16,
df = 6, 13, P < 0.0001; F = 3.13, df = 6, 13, P = 0.0139) but not deleterious parasitoids,
deleterious predators, predators, or herbivorous crop pests (F = 1.06, df = 6, 13, P = 0.4063;
F = 2.40, df = 6, 13, P = 0.1056; F = 1.86, df = 6, 13, P = 0.1029; F = 1.16, df = 6, 13, P =
0.3377). Upon closer observation we noted that blanket flower and fennel harbored higher
numbers of pollinators during the middle of our sampling dates while other flower species
were fed upon by approximately equal numbers of pollinators throughout the study.
Numbers of non-crop parasitoids observed feeding from flowers were low throughout the
entire study. Because we were able to identify probable causes leading to a significant effect,
data were averaged across both replication and date.
In 2005, no effect of replication was found for any of the feeding guilds. Week
significantly affected the number of pollinators and non-crop parasitoids observed on flowers
numbers of non-crop herbivores, herbivorous crop pests, predators, deleterious predators,
parasitoids, non-crop or deleterious parasitoids (F = 2.15, df = 4, 8, P = 0.1654; F = 0.50, df
= 3, 6, P = 0.6977; F = 0.83, df = 5, 10, P = 0.7181; F = 2.68, df = 5, 10, P = 0.0865; F =
2.03, df = 5, 10, P = 0.1599; F = 1.31, df = 5, 10, P = 0.3356). The number of pollinators
visiting flowers decreased steadily with the progression of weeks, likely because peak
flowering occurred at the beginning of the study and flower-production declined as weeks
passed. Non-crop parasitoids were observed feeding from flowers infrequently.
Discussion
In this study we were primarily interested in determining which flowering plants
provided floral food resources to beneficial insects. We regarded only two feeding guilds,
parasitoids and predators, as beneficial insects because of their ability to reduce numbers of
agricultural pests. We were also interested in recording all other insects feeding from floral
structures to determine whether or not crop pests fed from flowers and to separate insects
which may appear to be beneficial to farmers because they belong to the Order Hymenoptera.
The latter have species that may be deleterious because of their potential to reduce numbers
of pollinators or spiders through predation or parasitization (e.g. Pompilidae and
Chrysididae) (Triplehorn and Johnson 2005). We also recorded numbers of pollinators
which are beneficial to the farm but play no role in crop pest management.
Results from this study show that insects belonging to different feeding guilds
preferentially feed from different flower species. Although sampling was conducted in a
similar manner from year to year planting design was considerably different and plant
species observed differed making direct comparison of the two study years impossible.
exception of non-crop herbivores and non-crop parasitoids. Numbers of deleterious
parasitoids and predators, and herbivorous crop pests were not affected by flower species.
Additionally, some overall trends in the frequency of feeding visits made by beneficial
insects can be seen in both years. Fennel received the greatest number of feeding visits from
predators both years and in 2005 fennel was frequented most often by beneficial parasitoids.
In 2004, celery was visited most often by parasitoids. This study reinforces the observation
that umbelliferous flowers which have easily accessible nectaries are often frequented by
beneficial insects (Patt et al. 1997). Celery however only bloomed for three weeks in only
two of the three replicates. Additionally, celery is a biennial and therefore would unlikely be
a desirable beneficial insectary plant as growers would have to wait a full year for flowering
to commence. Fennel bloomed continuously and aggressively throughout both years of the
study.
In 2004, few insects were found feeding from buckwheat when all observations were
conducted at noon. Buckwheat tends to wilt in hot weather and does not produce nectar in
the afternoon (Lee and Heimpel 2003; Olson et al. 2005). By adding a morning observation
we were able to see that buckwheat was attractive to pollinators and predators after finding
the previous year that buckwheat attracted relatively low numbers of members of all feeding
guilds.
We found no significant effect of flower species on numbers of herbivorous crop
pests observed feeding from floral structures. Additionally, overall numbers of crop pests
feeding from flowers were low for both years. This does not mean, however that crop pests
did not feed from the flowers in this study. Time of day could have played an important role
(2004) observed crepuscular feeding habits of noctuid and sphingid moths and found that
moths fed most heavily from celosia flowers.
This study does not allow us to provide a definitive recommendation for beneficial
insect habitat to growers in North Carolina. In 2004, flowering was inconsistent across
replications and dates causing many gaps in the data. Siberian wallflower (Erysimum
hieracifolium L.) and dame’s rocket (Hesperis matronalis L.) were eliminated from data
analysis because they received so few feeding visits. This shows that these flowers are likely
poor choices as habitat planting to attract natural enemies in North Carolina. Other plants,
such as celery and cilantro (Coriandrum sativum L.) exhibited a short blooming period,
making them unsuitable insectary habitat plants as well. As was previously mentioned, the
biennial nature of celery is undesirable. In 2005 a similar problem was encountered with
Shasta daisy plants when blooming failed to commence the same season daisies were
planted. Fennel showed promising characteristics both phenologically and in its ability to
attract beneficial insects. Fennel, however, can be invasive and is listed on the California
Exotic Plant Pest List (1999). Fennel also causes contact and photodermatitis in humans and
should be handled only when wearing gloves (Simon et al. 1984). Because of the lack of
complete knowledge of biology and phenology of plants used in this study, future research
that is more exhaustive than the present study is needed. We hope that the current findings
can be a starting point for future observational studies of beneficial insect flower-feeding in
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Table 1.1 Plant species observed in each beneficial insect habitat flower strip. Goldsboro, NC 2004
Common Name Scientific Name Plant Family Weeks in Bloom Replicates in Bloom Cultivar Alfalfa Medicago sativa L. Fabaceae 2 1
Black-eyed Susan Rudbeckia hirta L. Asteraceae 7 2-3 Blanket flower Gaillardia pulchella Foug. Asteraceae 7 2-3 Blazing star Liatris spicata (L.) Willd. Asteraceae 3 1 Buckwheat Fagopyrum esculentum Polygonaceae 7 2-3
Moench
California poppy Eschscholzia californica Cham. Papaveraceae 5 3 Celery Apium graveolens L. Apiaceae 3 1-2 Cilantro Coriandrum sativum L. Apiaceae 3 1 Dame’s rocket Hesperis matronalis L. Brassicaceae 4 2-3
Fennel Foeniculum vulgare P. Mill. Apiaceae 7 3 ‘Smokey Bronze’ Purple prairie clover Dalea purpurea Vent. Fabaceae 2 1-2
Red clover Trifolium repens L. Fabaceae 7 3 Shasta daisy Leucanthemum x superbum Asteraceae 6 1-3
(J.W. Ingram) Berg. e x Kent.
Siberian wallflower Erysimum hieracifolium L. Brassicaceae 5 1-2 Tickseed Coreopsis lanceolata L. Asteraceae 5 1-2 Yarrow Achillea millefolium L. Asteraceae 7 3
--- 2005
Black-eyed Susan Rudbeckia hirta L. Asteraceae 3 3 ‘Indian Summer’ Buckwheat Fagopyrum esculentum Polygonaceae 3 3
Moench
Celosia Celosia cristata L. Amaranthaceae 5 3 ‘Cramer’s Crested Series Burgundy’
Fennel Foeniculum vulgare P. Mill. Apiaceae 7 3 ‘Smokey Bronze’ x Rubrum
Shasta daisy Leucanthemum x superbum Asteraceae 0 0 ‘Alaska’ (J.W. Ingram) Berg. e x Kent.
Table 1.2 List of insect families by feeding guild Feeding Guild Families Observed
Herbivore – Crop Pest Chrysomelidae, Coreidae, Curculionidae, Hesperiidae, Miridae, Papillionidae, Pentatomidae, Pieridae,
Scarabaeidae
Herbivore – Non-Crop Ctenuchidae, Geometridae, Mordellidae, Nymphalidae, Thyreocoridae
Parasitoid – Non-Crop Scoliidae, Tephiidae
Parasitoid – Beneficial Eulophidae, Figitidae, Tachinidae
Parasitoid – Deleterious Chrysididae
Pollinator Anthophoridae, Apidae, Halictidae, Megachilidae
Predator – Beneficial Anthocoridae, Cantharidae, Coccinellidae, Chrysopidae,
Lygaeidae, Sphecidae, Staphylinidae, Syrphidae, Vespidae
Table 1.3 Mean ± SD number of insects per two minutes in each feeding guild observed feeding from flowers 2 June – 4 August. Goldsboro, N.C. 2004
Plant Species Parasitoids Non-Crop Deleterious Herbivores Herbi vores Deleterious Predators Pollinators Parasitoids Parasitoids Non-Crop Crop Pests Predators
Black-eyed Susan 0.0 ± 0.0B 0.6 ± 1.1A 0.2 ± 0.6A 0.2 ± 0.4BC 0.1 ± 0.3A 0.0 ± 0.0A 0.5 ± 0.7BCD 1.3 ± 2.4CDE
Blanket flower 0.0 ± 0.0B 0.0 ± 0.0A 0.0 ± 0.0A 0.2 ± 0.7BC 0.2 ± 0.4A 0.0 ± 0.0A 0.2 ± 0.5CD 6.1 ± 4.6A
Buckwheat 0.0 ± 0.0B 0.5 ± 0.5A 0.0 ± 0.0A 0.2 ± 0.5BC 0.4 ± 1.0A 0.1 ± 0.2A 0.5 ± 1.0BC 0.0 ± 0.0F
California poppy 0.0 ± 0.0B 0.0 ± 0.0A 0.0 ± 0.0A 0.1 ± 0.3C 0.2 ± 0.4A 0.0 ± 0.0A 0.0 ± 0.0D 0.8 ± 0.8CDE
Celery 2.4 ± 3.6A 0.0 ± 0.0A 0.0 ± 0.0A 12.2 ± 18.6A 0.5 ± 0.6A 0.0 ± 0.0A 1.2 ± 1.3B 0.4 ± 0.9DEF
Clover 0.0 ± 0.0B 0.0 ± 0.0A 0.0 ± 0.0A 1.0 ± 3.15BC 0.5 ± 0.6A 0.0 ± 0.0A 0.4 ± 0.6CD 0.3 ± 0.2EF
Daisy 0.5 ± 1.4B 0.8 ± 1.2A 0.2 ± 0.6A 0.5 ± 0.7BC 0.6 ± 0.8A 0.0 ± 0.0A 0.9 ± 1.5B 1.5 ± 1.6CD
Fennel 0.5 ± 0.9B 0.2 ± 0.5A 0.1 ± 0.4A 0.4 ± 0.5BC 0.4 ± 0.8A 0.6 ± 1.2A 3.2 ± 2.6A 2.8 ± 3.4BC
Tickseed 0.0 ± 0.0B 0.0 ± 0.0A 0.0 ± 0.0A 0.7 ± 1.0BC 0.3 ± 0.5A 0.0 ± 0.0A 0.0 ± 0.0D 3.1 ± 2.4AB
Yarrow 0.1 ± 0.2B 0.4 ± 0.5A 0.1 ± 0.4A 2.0 ± 3.2B 1.6 ± 1.5A 0.0 ± 0.0A 0.7 ± 1.0BC 1.8 ± 1.97C
Table 1.4 Mean ± SD number of insects per two minutes in each feeding guild observed feeding from flowers1 21 June – 16 August. Goldsboro, N.C. 2005
Plant Species Time of Parasitoids Non-Crop Deleterious Herbivores Herbivores Deleterious Predators Pollinators Day Parasitoids Parasitoids Non-Crop Crop Pests Predators
Fennel 9:30 1.0 ± 1.3A, A 0.1 ± 0.3AB, AB 0.0 ± 0.3A, A 0.2 ± 0.4A, A 0.3 ± 0.5A, A 0.2 ± 0.5A, A 3.0 ± 2.4AB, AB 2.5 ± 2.5B, BC
Buckwheat 9:30 0.2 ± 0.6B, A 0.8 ± 1.5A, A 0.1 ± 0.3A, A 0.1 ± 0.2A, A 0.2 ± 0.4A, A 0.0 ± 0.0A, A 2.0 ± 1.8BC, B 5.1 ± 3.2A
Yarrow 9:30 0.0 ± 0.0B, A 0.0 ± 0.0B, B 0.0 ± 0.1A, A 0.1 ± 0.3A, A 0.3 ± 0.4A, A 0.0 ± 0.0A, A 0.2 ± 0.4F, F 1.1 ± 1.4CD, CD
Celosia 9:30 0.0 ± 0.0B, A 0.0 ± 0.0B, B 0.0 ± 0.0A, A 0.0 ± 0.0A, A 0.2 ± 0.3A, A 0.0 ± 0.0A, A 1.5 ± 0.8C, D 0.9 ± 1.0D, D
Black-eyed Susan 9:30 0.0 ± 0.0B, A 0.0 ± 0.1B, B 0.0 ± 0.0A, A 0.0 ± 0.0A, A 0.0 ± 0.0A, A 0.0 ± 0.0A, A 0.4 ± 0.6F, F 4.3 ± 2.2A
--- Fennel 12:00 0.3 ± 0.4A, B 0.7 ± 0.9AB, AB 0.0 ± 0.0A, A 0.2 ± 0.5A, A 0.1 ± 0.3A, A 0.4 ± 0.5A, A 3.8 ± 2.0A, AB 2.2 ± 1.8BC, B
Buckwheat 12:00 0.0 ± 0.0A, B 0.9 ± 1.8A, A 0.3 ± 0.5A, A 0.1 ± 0.2A, A 0.0 ± 0.0A, A 0.0 ± 0.1A, A 1.2 ± 1.2E, D 0.7 ± 1.0D, D
Yarrow 12:00 0.0 ± 0.1A, B 0.1 ± 0.3B, B 0.2 ± 0.4A, A 0.4 ± 0.7A, A 0.4 ± 0.4A, A 0.0 ± 0.0A, A 0.7 ± 1.5EF, F 2.6 ± 2.8BC, B
Celosia 12:00 0.0 ± 0.0A, B 0.1 ± 0.3B, B 0.0 ± 0.0A, A 0.1 ± 0.2A, A 0.2 ± 0.3A, A 0.0 ± 0.0A, A 2.0 ± 1.7C, BC 1.0 ± 1.2CD, CD
Black-eyed Susan 12:00 0.0 ± 0.0A, B 0.2 ± 0.3B, B 0.1 ± 0.2A, A 0.0 ± 0.0A, A 0.0 ± 0.0A, A 0.0 ± 0.0A, A 0.1 ± 0.2F, F 2.3 ± 1.2B, B
Relative Attractiveness of Habitat Plantings to Microhymenoptera
BROOKE ELLEN WITTING
Department of Entomology, College of Agriculture and Life Sciences
Abstract
Flowering habitat is used in cropping systems to provide a food source in the form of
nectar or pollen to natural enemies of agricultural insect pests. This study was conducted in
August 2005 to determine the relative attractiveness of floral habitat to three families of
microhymenopteran egg parasitoids: Mymaridae, Scelionidae, and Trichogrammatidae.
Habitat plants were yarrow (Achillea millefolium L.),celosia (Celosia cristata L.),
buckwheat (Fagopyrum esculentum Moench), fennel (Foeniculum vulgare P. Mill.), daisy
(Leucanthemum x superbum (J. W. Ingram) Berg. ex Kent.), and black-eyed Susan
(Rudbeckia hirta L.). Non-flowering crabgrass (Digitaria sp. Haller) served as a control.
Sticky traps were used to monitor microhymenoptera and were placed at three heights:
flower height, 0.5 times flower height, and 1.5 times flower height. Flower heads were
removed from half of each plot and traps were placed in the center of each subplot. Trapped
microhymenoptera were counted with the expectation that greater numbers would be trapped
in subplots with flowers intact at flower height if flowers were indeed attractive. Results
from this experiment show that flower species and height affected all three families of
microhymenoptera but flower removal only affected scelionids. At flower height, scelionids
were trapped in greater abundance in celosia plots at flower height in flowers-present versus
flowers-removed treatments. Trichogrammatids were trapped in greatest abundance at 0.5
times flower height in un- mowed crabgrass plots and mymarids were most abundant at 0.5
times flower height in black–eyed Susan plots. Our results indicate that habitat plantings
may attract microhymenoptera but that flowers themselves do not appear to be responsible
Introduction
A wide variety of predators and parasitoids are relied upon for biological control of
insect crop pests. Microhymenopteran parasitoids, in particular, can play a crucial role in
reducing crop pest numbers. Egg parasitoids can be especially important since pests are
killed before feeding-damage to crops can occur. Numerous studies have been conducted
using direct observation to determine food preferences of predators and parasitoids (Jervis et
al. 1993; Carreck and Williams 1997; Al-Doghairi and Cranshaw 1999; Colley and Luna
2000). However, microhymenopteran parasitoids are minute, making direct observation of
feeding very difficult.
Parasitic microhymenoptera have short mouthparts. Because of this, floral
architecture and nectar accessibility play an important role in determining the attractiveness
and suitability of flowering habitat to microhymenoptera. Plants in the carrot family
(Apiaceae) and the buckwheat family (Polygonaceae) have been determined to successfully
provide resources to microhymenoptera and other short-tongued beneficial insects such as
hoverflies, because of their small florets and exposed nectaries (Lövei et al. 1993; Proctor et
al. 1996; Tooker and Hanks 2000). In a study examining floral architecture prefe rences of
two microhymenopteran parasitoids in the family Eulophidae Patt et al. (1997) found
parasitoids foraged more effectively on flowers with open, easily-accessible nectaries.
Maingay et al. (1991) collected hundreds of individuals of numerous species of
entomophagous and parasitic hymenoptera feeding from sweet fennel (Foeniculum vulgare
P. Mill. var. dulce Battandier & Trabut ) (Apiaceae). Stephens et al. (1998) found increased
parasitism and higher numbers of a braconid parasitoid in orchard understories sown with
experiment, English-Loeb et al. (2003) found higher egg parasitism by Anagrus parasitoids
(Hymenoptera: Mymaridae) caged on buckwheat flowers with flowers present than those
caged on buckwheat with inflorescences removed. Irvin et al. (2000) found parasitoids to be
seven times more abundant in buckwheat plantings with flowers present than in those with
flowers removed. No difference was found between buckwheat plants with flowers removed
and an herbicide-treated control, indicating that floral resources rather than vegetative
properties of buckwheat were responsible for attraction of parasitoids to the plants.
However, not all studies using flowering plants to enhance numbers of
microhymenopteran parasitoids have proved successful. For example, Berndt et al. (2002)
examined abundance of two leafroller parasitoids in buckwheat plantings compared to grass
and clover controls. No difference in abundance of Glyptapanteles demeter (Wilkinson)
(Hymenoptera: Braconidae) was found and only significantly higher numbers of male
Dolichogenidea tasmanica (Cameron) (Hymenoptera: Braconidae) parasitoids were trapped
in buckwheat plantings. In a review by Heimpel and Jervis (2005) an increase of parasitism
was observed in only seven out of twenty studies comparing floral habitat to controls.
Additionally, only one out of the twenty studies showed a decline in pest numbers. This
indicates that even if microhymenoptera are attracted to flowering habitat, a decrease in pest
density is not guaranteed.
The present study was conducted to indirectly measure the relative attractiveness of
different flowering plants to microhymenopteran egg parasitoids in the families Mymaridae,
Scelionidae, and Trichogrammatidae in North Carolina. Plants for this study were chosen
Materials and Methods
Research site. Research was conducted on the Small Farm Unit at the Center for Environmental Farming Systems near Goldsboro, N.C. The Small Farm Unit is a highly
diverse organic farm approximately 6.07 ha in size. A wide variety of commodities are
grown at the Small Farm Unit including vegetable, flower, and small fruit crops. Some
livestock production, including chickens, turkeys, and goats also occurs on the Small Farm
Unit.
Experimental design. Measurements of the abundance of microhymenoptera in habitat plantings were collected from three replicates, each measuring 56.4 x 2.7 m, divided
into seven 6.1 x 2.7 m plots. Replicates were separated from one another by an average
distance of 48.2 m. Plots were laid out in the following order from the northeast to the
southwest: celosia (Celosia cristata L.), fennel (Foeniculum vulgare P. Mill.), yarrow
(Achillea millefolium L.), black-eyed Susan (Rudbeckia hirta L.), buckwheat (Fagopyrum
esculentum Moench), and daisy (Leucanthemum x superbum (J. W. Ingram) Berg. ex Kent.).
A plot at southwest end of each replicate dominated by naturally occurring crabgrass
(Digitaria sp. Haller) served as a control.
Celosia, black-eyed Susan, and daisy plants were transplanted into plots in three rows
with plants spaced 30.5 cm apart and 46 cm between each row using hand trowels and bulb
diggers on 25 May, 2005. Buckwheat (Jeffrey's Seed Co., 1608 US 117 South, Goldsboro,
NC 27503) was hand-seeded at a rate of 56.04 kg/ha. Fennel and yarrow plants were planted
in 2003 as previously described (Chapter 1).
All flower heads were removed from half of each treatment plot using pruning shears
determine from which side of the plot to remove plants. Flower removal prior to bud-break
and mowing occurred for the remainder of the study.
Plant management. Celosia, black-eyed Susan, and daisy plants (See Table 1.1 for cultivars) were grown in the Biological Control greenhouse at North Carolina State
University, Raleigh, NC. Heating and ventilation set points were 21.1º C and 26.7º C,
respectively. Seeds (Germania, 5978 N Northwest Hwy, PO Box 31787, Chicago, IL
60631-0787) were planted in 96-cell round plug trays (3.8 x 3.9 cm, Hummert International, 4500
Earth City Expressway, Earth City, MO 63045) filled with moistened Metro-Mix 200 potting
soil (Scotts-Sierra Horticulture Products Co., The Scotts Company, 1411 ScottsLawn Rd.,
Marysville, OH 43041) in late March of 2005. Plants were watered as needed with a misting
bed and/or hand watering. Trays were placed under high intensity metal halide lights with an
11 h photophase. Photophase was extended to 16 h on 22 April, 2005. Plants were
transplanted to 473 ml plastic cups (Kmart Corporation, Troy, MI 48084) with a drainage
hole drilled in the bottom using a 1.3 cm drill bit when roots were established and
aboveground portions were of sufficient size.
Prior to transplanting, plots were tilled and celosia, black-eyed Susan, and daisy plots
as well as the borders surrounding all plots were covered with woven black plastic ground
cover (Wyatt-Quarles Seed Company, 730 Hwy 70 West, Garner, NC 27529) secured with
landscape anchor pins (DuPont™ Garden Products™, Chestnut Run Plaza, Bldg. 728, PO
Box 80728, Wilmington, DE, 19880-0728) to suppress weeds and preserve soil moisture.
Plants were planted through holes cut in the ground cover. Watering occurred as needed and
weeds were managed with hand-pulling inside plots and mechanical mowing around plots.
of 19 mm diameter PVC pipe spray painted with yellow plastic enamel (The Valspar
Corporation, Wheeling, IL 60090) and wrapped with tanglefoot-coated clear acrylic sheets
(Great Lakes IPM, 10220 Church Rd. NE, Vestaburg, MI 48891-9746). In each subplot,
traps were placed on a single stake at three heights: 0.5 the height of flowers, flower height,
and 1.5 times flower height. Traps were secured to plastic stakes and were changed twice
weekly from 9 August to 16 August, 2005.
Immediately following collection, traps were returned to the laboratory where
tanglefoot-coated acrylic sheets were removed from PVC sections, sandwiched between two
sheets of clear plastic wrap (Kmart Corporation, Troy, MI 48084), and placed in plastic
freezer bags (1 qt., Hefty®, Pactiv Corp., 1900 W Field Ct., PO Box 5032, Lake Forest, IL
60045) for storage in a freezer at -20º C. Using a dissecting microscope (Leica, Wild MZ8,
Leica Microsystems GmbH, Ernst-Leitz-Strasse 17-37, 35578 Wetzlar) the number of
individuals in the families Mymaridae, Scelionidae, and Trichogrammatidae on each sheet
was recorded.
Data analysis. Abundance data were square root transformed prior to analyses. Data were analyzed to determine the effects of flower species, flower removal, and trap height on
abundance of microhymenoptera in habitat plantings using general linear models (PROC
GLM) and least significant difference (LSD) tests of means (SAS, 2003). Type III Sums of
Squares are presented in Appendix 2.1-2.2 and t-groupings from LS D tests are presented in
Table 2.1.
Results
Flower species significantly affected abundance of mymarids and trichogrammatids
1.83, df = 5, 10, P = 0.1947) (Appendix 2.1). Height (F = 21.47, df = 2, 44, P < 0.0001; F =
25.51, df = 2, 44, P < 0.0001; F = 8.25, df = 2, 44, P = 0.0009) and the interaction between
flower species and height played a significant role in abundance of mymarids, scelionids, and
trichogrammatids (F = 7.24, df = 10, 44, P < 0.0001; F = 6.69, df = 10, 44, P < 0.0001; F =
4.17, df = 10, 44, P = 0.0004). The interaction between flower species and flower removal
significantly affected trichogrammatids (F = 7.16, df = 5, 12, P = 0.0026) but not mymarids
or scelionids (F = 0.56, df = 5, 12, P = 0.7280; F = 1.35, df = 5, 12, P = 0.3104). Flower
removal and the interaction between flower removal and height significantly affected
abundance of scelionids (F = 6.76, df = 1, 12, P = 0.0232; F = 6.20, df = 2, 44, P = 0.0042).
Flower removal and the interaction between flower removal and height did not significantly
affect abundance of mymarids (F = 1.62, df = 1, 12, P = 0.2266; F = 2.26, df = 2, 44; P =
0.1167) or trichogrammatids (F = 0.18, df = 1, 12, P = 0.6818; F = 0.41, df = 2, 44, P =
0.6672). There was a significant three way interaction between flower species, flower
removal, and height for scelionids and trichogrammatids (F = 2.64, df = 10, 44, P = 0.0130;
F = 2.28, df = 10, 44, P = 0.0298), but not for mymarids (F = 1.69, df = 10, 44, P = 0.1123).
Among the different heights, a significant flower effect was found for mymarids,
scelionids, and trichogrammatids at height 2 (flower height ) (F = 5.08, df = 5, 10, P =
0.0141; F = 4.70, df = 5, 10, P = 0.0182; F = 5.78, df = 5, 10, P = 0.0092) and height 1 (0.5
times flower height) (F =12.55, df = 5, 10, P = 0.0005; F = 3.24, df = 5,10, P = 0.0536; F =
22.38, df = 5, 10, P < 0.0001) (Appendix 2.1). At the height 3 (1.5 times flower height),
there was a significant flower effect on abundance of trichogrammatids (F = 5.58, df = 5, 10,
P = 0.0103) but not on abundance of mymarids (F = 2.56, df = 5, 10, P = 0.0965) or
Discussion
Abundance of microhymenoptera caught on sticky traps was used as an indirect
indicator of relative attractiveness of each plant species to the three parasitoid families
studied. The assumption was made that if flowers were attractive to microhymenoptera, a
greater number would be caught at height 2 (the height of flower heads) in the subplots
where flowers had not been removed. Crabgrass was chosen as the control for this study
because it offered a vegetative habitat without flowers. It was assumed that if flowers were
attractive, more microhymenoptera would be caught in plots containing flowering habitat
than in non- flowering controls.
Each microhymenopteran family responded differently to the plants in this study
(Table 2.1). Mymarids were found in greatest abundance at height 1 in black-eyed Susan
plots. Scelionids were most abundant in celosia plots at height 2. The greatest number
trichogrammatids were trapped in crabgrass control plots both at height 1 and height 3. None
of the flowers determined to attract microhymenoptera belong to the families Apiaceae or
Polygonaceae. These findings are significant because both fennel and buckwheat have been
heralded as suitable beneficial insect habitat (Maingay et al. 1991; Stephens et al. 1998;
Irvin. et al. 2000; English- Loeb et al. 2003). Similar to the present findings, past work on the
Small Farm Unit found abundance and diversity of natural enemies sampled from various cut
flower and herb species to be lowest in plots containing pure stands of fennel and highest in
celosia (Forehand 2004).
Little evidence was found in this study that flower removal affected the number of
wasps caught on traps. For the majority of the plant species tested, numbers of trapped
subplots where flowers had been removed. Only scelionids were found in greater abundance
at flower height in celosia plots where flowers remained intact (Table 2.1). This finding was
similar to that of Rebek et al. (2005) who found that the removal of inflorescences from four
species of flowering plants in an ornamental landscape had no effect on abundance of natural
enemies collected on sticky cards. Both these studies contradict results of Irvin et al. (2000)
who found greater abundance of the leafroller parasitoid Dolichogenidea tasmanica in
buckwheat plantings with flowers present than in plantings where flowers had been removed
indicating an attraction to floral structures.
Overall, the abundance of sampled microhymenoptera in this study was not different
in flower plots compared to control (crabgrass) plots. Scelionids and mymarids were found
in greater numbers in a few plots containing flowering plants than in the control plots. Of
these plots, mymarids were solely found in higher numbers halfway below the flower of
black-eyed Susan and scelionids in greater abundance in celosia plots at height 2 (Table 2.1).
These findings suggest the flowers themselves were not attractive to mymarids.
English-Loeb et al. (2003) found parasitism by mymarids to increase in the presence of buckwheat
flowers. However, mymarids were caged on buckwheat putting them in close proximity to
flowers. In the field, mymarids may not be able to locate flowers because of their reduced
wings. Scelionids showed preferential attraction to celosia plantings at flower height
indicating a possible attraction to floral structures. Overall, scelionids are larger in body size
and have more well-developed wings than mymarids or trichogrammatids. This could allow
scelionids to preferentially locate floral food resources due to greater flight ability. At height
2, trichogrammatids were most abundant in yarrow plots where flowers had been removed
but were also highly abundant in mowed crabgrass control plots and buckwheat plots where
flowers had been removed (Table 2.1). This shows that while trichogrammatids appeared to
be attracted to some habitats, flowers were clearly not responsible for this attraction.
Future field studies could be conducted to investigate which vegetative qualities of
plants, rather than flowers, determine relative attraction to microhymenoptera. If vegetative
habitat is attractive to different microhymenoptera, it would be useful to determine which
habitats are preferred. In the current study, mean numbers of trichogrammatids were
significantly greater within the canopy (height 1) of un- mowed crabgrass plots than in the
canopy of any other plant species studied (Table 2.1). Using paper models of plant foliage
Lukianchuk and Smith (1997) determined that female T. minutum Riley had a greater
foraging success on simple rather than complex surfaces. It may be that the vegetative
qualities of grass in this study exhibited a less complex structure than the foliage of the
flowering plants. Tric home-density on plant surfaces could have played a role in preference
of some plants over others. Keller (1987) determined that walking speed of T. exiguum was
influenced by leaf-trichome form and density, with less-densely pubescent leaves permitting
the fastest walking speeds. Measures of trichome-density and type are generally used to
evaluate host- finding ability of parasitoids but could be important if trichomes impede
location of food sources. Quantification of foliar trichomes could also be valuable since
trichomes can provide shelter to microhymenoptera (Cortesero et al. 2000). In the present
study, mymarids were found in greatest abundance in black-eyed Susan plots at height 1
regardless of flower presence or absence. Black-eyed Susan and celosia in our plots were
similar with regard to height, leaf size and shape, amount of foliage, and canopy closure.
